One type of fluid used in oil and gas well drilling operations is a "completion fluid." A completion fluid is pumped down a well after drilling is completed during the "completion phase." Drilling mud typically is removed from the well using "completion fluid," which typically is a clear brine. Then, the equipment required to produce fluids to the surface is installed in the well. A completion fluid must have sufficient viscosity to maintain the filter cake and sufficient elasticity to maintain a suspension of bridging or weighing agents.
Other types of fluids used during drilling operations are "drill-in" and "drilling" fluids. A drill-in fluid is pumped through the drill pipe while drilling through the "payzone," or the zone believed to hold recoverable oil or gas. A drilling fluid is used to drill a borehole through the earth to reach the payzone. Typically a drilling mud is circulated down through the drill pipe, out the drill bit, and back up to the surface through the annulus between the drill pipe and the borehole wall. The drilling fluid has a number of purposes, including cooling and lubricating the bit, carrying the cuttings from the hole to the surface, and exerting a hydrostatic pressure against the borehole wall to prevent the flow of fluids from the surrounding formation into the borehole.
Fluids in which water is the continuous phase provide the fastest drilling rates, and are ecologically favored over fluids in which oil is the continuous phase. Unfortunately, water does not have the viscosity profile suitable to carry drill solids to the surface from any significant depth. In addition, the walls of a wellbore frequently are composed at least in part of shale. When exposed to water, many shales swell, slough, or spall to the extent that they may even prevent further operation of the wellbore. Bits of shale may slough off during gravel transport, mix with the gravel, and drastically reduce the effectiveness of well completion by choking off the permeability of the gravel pack, as well as by causing screen blinding.
Water-soluble polymers, such as partially hydrolyzed polyacrylamides ("PHPA's"), starches, derivatized starches, gums, derivatized gums, and cellulosics, typically are used to thicken water-base fluids, and in part to synergistically stabilize shale. The water-soluble polymers provide the viscosity necessary to lift drilled solids from the wellbore, and tend to provide extremely fast drilling rates. However, the shear stresses in the immediate vicinity of the drill bit are high enough to degrade most water-soluble polymers, resulting in lowered viscosity.
Polyglycols are known to stabilize the water-soluble polymers, and to stabilize shale. Monomers of propylene oxide (specifically 1,2-propylene oxide) and butylene oxide (specifically 1,2-butylene oxide) typically are used to manufacture polyglycols for such uses because these monomers increase the hydrophobic character of the polyglycols and lower their cloud point temperature in water. Unfortunately, the use of 1,2-alkylene oxide monomers results in the introduction of short alkyl side groups into the resulting polyglycol, which severely reduces the biodegradability of the polyglycol.
It would be desirable if more biodegradable polyglycols were developed for use during drilling operations.